This invention relates to optical waveguides formed in integrated circuit (IC)-like structures and positioned in interconnect layers of the IC-like structure. More particularly, this invention relates to a new and improved optical waveguide formed with a graded index of refraction in an IC-like structure. In addition, the present invention relates to a new and improved method of fabricating a graded index of refraction waveguide in an IC-like structure using damascene fabrication process steps that are typically employed in the fabrication of electrical integrated circuits.
The ongoing evolution of microcircuit design has focused on the speed and size of electrical integrated circuit (IC) components, typically in a silicon chip. IC designers continuously strived to make the IC faster while taking up less chip space. Currently, interconnection technology is considered as one of several areas that may be advanced to both increase the speed of the IC and to decrease the size of the chip. For instance, since most of the conductors that interconnect various functional components on the chip are made of metal and carry electrical signals, advances are being made in various metal compositions that can carry similar signals at a faster speed but which are smaller and thus consume less space.
Optical signals carried by waveguides are sometimes considered as replacements to the more common electrical signals carried by metal conductors. Optical signals allow the IC to operate more quickly or at a higher speed, and unlike electrical signals, optical signals are usually not susceptible to noise and interference. In general, optical conduction and reduced susceptibility to noise and interference obtain increased speed in data transmission and processing.
Furthermore, due to the coherent nature of laser optical signals and their reduced susceptibility to noise, many more optical signals can be routed in one waveguide or layer of waveguides than is possible using conventional electrical signal interconnect conductors. Therefore, an IC-like structure incorporating optical interconnect waveguides may have fewer waveguides and consume less space.
One typical type of optical interconnection between two IC components comprises a single waveguide or channel between the two components. In general this single waveguide is a straight conductive path between conversion devices which convert electrical signals to optical signals and convert optical signals to electrical signals.
Another type of controllable optical interconnect is called a xe2x80x9crailtap.xe2x80x9d A railtap comprises a first conversion device that converts an electrical signal from a first IC component to an optical signal, an interconnect waveguide that conducts the optical signal from the first conversion device to a second conversion device, where the second conversion device converts the optical signal to an electrical signal and applies it to the second IC component. Upon receiving an electrical signal from the first component, the railtap diverts an optical light signal from a light source waveguide into the interconnect waveguide. An active waveguide polymer is connected to electrodes, and the electrodes create an electric field about the active waveguide polymer, causing a change in the index of refraction of the polymer, usually making it loser to the index of refraction of the source waveguide. When the index of refractions of the railtap and the source waveguide are similar, light is refracted from the source into the railtap polymer. Light is thereby transmitted selectively through the interconnect waveguide toward the second component as a result of applying the electric field to the electrodes on the active waveguide polymer.
The typical waveguide is formed of light transmissive material which is surrounded by an opaque cladding material. Optical signals propagate through the channel, guided by the cladding material. As the optical signals propagate through a particular waveguide, the signals impinge on the cladding material. If the index of refraction of the cladding material is lower than the index of refraction of the material within channel, the majority of the impinging light signal reflects from the cladding material and back towards the center of the channel. Thus the signal propagates through the channel as a result of reflection at the interface of the cladding material.
On the other hand, if the index of refraction of the cladding material is equal to or greater than the channel material, the impinging light signal tends to refract into the cladding material, thus drawing some or all of the optical power of the light signal out of the waveguide. As more light is drawn out of the waveguide, the intensity of the signal received from the waveguide is reduced. An ideal, lossless waveguide propagates an optical signal without losing any signal intensity through refraction.
Typical waveguides used as optical interconnects in IC-like structures comprise a singular channel material having a predetermined index of refraction which is greater than the index of refraction of the cladding material surrounding the channel. Consequently, losses in signal intensity are minimized because the refraction of light energy into the cladding material is minimized due to the lower index of refraction of the cladding material compared to the index of refraction of the channel material. However, the single index of refraction of the channel material eliminates the ability to tailor the waveguide to a lossless or near-lossless condition.
Another drawback associated with fabricating single index of refraction waveguides relates to what is called xe2x80x9cdishing.xe2x80x9d During a typical IC damascene process, material is first deposited in a trench or hole and then polished so that the upper surface of the deposited material is flush with the upper surface of the surrounding material. The different polish rates of the different materials often result in an increased portion of the deposited material being removed from the trench. Consequently, the upper surface of the material filling the trench is not coplanar with the upper surface of the surrounding material, but instead is somewhat concave (viewed top-down), reducing the cross-sectional area of the deposited material. The somewhat concave surface may adversely interfere with signal propagation by causing unwanted reflection and lens effects.
The dishing problem is generally worse for wider trenches than for narrow trenches. Unfortunately, narrowing the trench is not an acceptable solution since optical waveguides require specific cross sectional area dimensions to accommodate an integer number of wavelengths of the optical signal conducted.
It is also known to form graded index of refraction waveguides in IC-like structures. However, the structure of the graded index of refraction waveguide and the process by which it is fabricated in the IC-like structure are unusual, and to a certain degree, are difficult to utilize effectively. For example, the waveguide material must first be formed, and then the support structure for it must be eroded, dissolved or otherwise removed, leaving the waveguide material suspended and fully exposed in free space. A coating material is then vapor deposited in the three dimensions surrounding the waveguide material. Heat is applied, which causes the vapor deposited material to penetrate into the waveguide material from the exterior. The penetration of the vapor deposited material into the exterior of the waveguide material from all sides modifies the index of refraction of the exterior of the waveguide material, thereby creating a graded index of refraction waveguide.
While this prior process is effective in creating a graded index of refraction, the steps of eroding, dissolving or otherwise removing the support structure to expose the waveguide material in free space are unusual and difficult to accomplish. In general these steps are not typically applied in fabricating IC structures. Furthermore, this type of fabrication process cannot be applied in all types of IC-like structures because the supporting material cannot be removed in some IC-like structures, and the deposition and heating of the material cannot be accomplished with some types of IC-like structures. These steps will damage or adversely influence the other components formed on the structure.
It is with respect to these and other considerations that the present invention has evolved.
An aspect of the present invention is to form an improved optical waveguide having a graded index of refraction within an integrated circuit (IC). Another aspect is to form an optical waveguide with higher transmission efficiencies and less losses. A further aspect is forming a graded index of refraction optical waveguide in an IC-like structure using the typical process steps used to fabricate a typical IC. Yet another aspect is to fabricate a graded index of refraction waveguide while minimizing the effects of dishing.
In accordance with these and other aspects, the present invention relates to a graded index of refraction waveguide in an integrated circuit-like structure having a substrate. The waveguide includes at least one layer of dielectric material positioned above the substrate which defined a trench having side walls. A refractive layer of optically transmissive material is formed within the trench adjoining the side walls. A core of optically transmissive material is also formed in the trench adjoining the refractive layer. The materials of the core, refractive layer and dielectric material are selected so that the index of refraction of the core is greater than the index of refraction of the refractive layer, than the index of refraction of the refractive layer is greater than the index of refraction of the dielectric material.
In accordance with yet other aspects, the invention also relates to a method of fabricating a graded index of refraction optical waveguide in interlayer dielectric material located above a substrate an integrated circuit-like structure. The method includes the steps of forming an elongated trench in the dielectric material, forming a U-shaped refractive layer of optically transmissive material in the trench, forming a core of optically transmissive material within the U-shaped refractive layer, selecting the core material to have a higher index of refraction than the material of the refractive layer, and selecting the material of the refractive layer to have a higher index of refraction than the dielectric material.
In accordance with still further aspects, the invention also relates to a method of fabricating a graded index of refraction optical waveguide in interlayer dielectric material located above the substrate in an integrated circuit-like structure. A refractive layer of optically transmissive material is formed surrounding a core of optically transmissive material within a trench in the dielectric material. The material of the core is selected to have a higher index of refraction than the material of the refractive layer, and the material of the refractive layer is selected to have a higher index of refraction than the dielectric material.
Selecting the index of refraction of the refractive layer to be less than that of the core but greater than that of the dielectric material creates a gradation in the indices of refraction which refracts a greater amount of the light energy into the core, rather than allowing the light energy to be lost into the surrounding dielectric material. The efficiency of the waveguide is thereby increased and the losses are decreased.
Other preferred features of the present invention are described in this paragraph. The core may be surrounded on all but one side by the refractive layer. The refractive layer may be U-shaped to surround the core except on the one side. A cap of refractive material having essentially the same index of refraction as the U-shaped refractive layer extends across the one side of the core between ends of the U-shaped refractive layer. A second refractive layer of optically transmissive material in addition to the first refractive layer may be formed in the trench between the core and the first refractive layer. The second refractive layer has an index of refraction which is less than the index of refraction of the core and greater than the index of refraction of the first refractive layer. Caps of the refractive material extend across the one side of the core between ends of one of the first or second U-shaped refractive layers to form encircling refractive layers surrounding the core. The first and second refractive layers are preferably formed by deposition in a self aligned manner within the trench.
The fabrication method preferably employs a damascene technique of forming the trench into the dielectric material from an upper exposed surface of the dielectric material and thereafter progressing in the self-aligning and filling process steps as described. Consequently, the graded index of refraction waveguide may be formed in an IC-like structure using the typical, well-known and reliable fabrication techniques employed in other types of IC fabrication, particularly when the IC-like structure includes both electrical and optical components.
A more complete appreciation of the present invention and its scope, and the manner in which it achieves the above noted improvements, can be obtained by reference to the following detailed description of presently preferred embodiments of the invention taken in connection with the accompanying drawings, which are briefly summarized below, and by reference to the appended claims.